An inkjet printing system typically includes one or more printheads and their corresponding ink supplies. Each printhead includes an ink inlet that is connected to its ink supply and an array of drop ejectors, each ejector consisting of an ink pressurization chamber, an ejecting actuator and a nozzle through which droplets of ink are ejected. The ejecting actuator may be one of various types, including a heater that vaporizes some of the ink in the pressurization chamber in order to propel a droplet out of the orifice, or a piezoelectric device which changes the wall geometry of the chamber in order to generate a pressure wave that ejects a droplet. The droplets are typically directed toward paper or other recording medium in order to produce an image according to image data that is converted into electronic firing pulses for the drop ejectors as the recording medium is moved relative to the printhead.
A common type of printer architecture is the carriage printer, where the printhead nozzle array is somewhat smaller than the extent of the region of interest for printing on the recording medium and the printhead is mounted on a carriage. In a carriage printer, the recording medium is advanced a given distance along a media advance direction and then stopped. While the recording medium is stopped, the printhead carriage is moved in a direction that is substantially perpendicular to the media advance direction as the drops are ejected from the nozzles. After the carriage has printed a swath of the image while traversing the recording medium, the recording medium is advanced; the carriage direction of motion is reversed, and the image is formed swath by swath.
The ink supply on a carriage printer can be mounted on the carriage or off the carriage. For the case of ink supplies being mounted on the carriage, the ink tank can be permanently integrated with the printhead as a print cartridge so that the printhead needs to be replaced when the ink is depleted, or the ink tank can be detachably mounted to the printhead so that only the ink tank itself needs to be replaced when the ink tank is depleted.
One type of detachable ink tank includes a porous member (also called a wick or scavenger member) at the ink supply port. The printhead inlet port can include a standpipe, for example, with a filter member at its inlet end. When the ink tank is mounted onto the printhead, the ink tank wick is held in contact with the filter member on the standpipe of the printhead inlet port. The ink supply port of the ink tank includes a rim having a face that seals against a gasket surrounding the inlet port of the printhead when the ink tank is installed. The gasket seal provides a substantially airtight ink pathway from the ink tank to the printhead. Once the printhead is primed so that liquid ink fills the various ink passageways between the wick and the nozzles on the printhead, capillary action provides the force necessary to supply the ink to the nozzles as needed for printing. Such an ink tank facilitates easy and clean installation onto the printhead
Some types of ink tanks also include capillary media such as felt or foam that is used to retain ink inside the ink tank and provide a slight negative ink pressure so that ink does not drip out of the nozzles of the printhead. This ink-retaining capillary media thus serves as a pressure regulator and provides ink to the wick at the ink supply port.
It has been found that pigment particles in a pigmented ink can settle out in ink tank designs where ink is stored in a capillary media pressure regulator, partly due to the restriction of motion of pigment particles within the small passages of the capillary media, as described in more detail in US Patent Application Publication Number US20090309940. Such settling of pigments particles, especially for larger pigment particles (e.g. larger than 30 nanometers), can result in defective images during the printing process. As a result, an ink tank using capillary media to store ink can lead to a limitation in pigment particle size that can be used. Such a limitation can be disadvantageous because such larger particles can be beneficial for providing higher optical density in printed regions.
A different type of pressure regulator for an ink tank is a bag (or flexible wall) with a spring that provides pressure regulation for a supply of liquid ink within a reservoir of the ink tank. Such ink tanks can have less tendency for settling out of pigment particles than for the case of ink stored in capillary media. In addition, as disclosed in U.S. Pat. No. 7,086,725, an ink tank having a flexible wall or a bag and a spring for pressure regulation can provide ink from the reservoir more efficiently (i.e. less ink trapped in the depleted reservoir) than an ink tank using capillary media ink storage to perform pressure regulation.
In conventional ink tanks having a spring-biased flexible wall, no vent is provided to the ink-filled reservoir. In order to provide pressure regulation for nearly empty reservoirs as well as nearly full reservoirs, it is necessary in such ink tanks for the spring-biased flexible wall to be able to collapse substantially completely in the case of a nearly empty ink reservoir. This requires the flexible wall to have a high degree of flexibility. However, it is also desirable for the flexible wall to have low permeability to air, in order to keep air from passing through the flexible wall and being absorbed into the ink, which can result in air bubbles in the printhead. However, additional layers for low air permeability can reduce the degree of flexibility of the flexible wall. U.S. Pat. No. 6,773,099 discloses an ink tank with a spring-biased flexible wall and a one-way valve to allow air bubbles to enter the reservoir for maintaining a stable negative pressure as ink is used. However, typically such a one-way valve has a substantial cracking pressure at which the valve opens, leading to fluctuations in the regulated pressure, as the negative pressure in the reservoir needs to build up sufficiently to open the one-way valve. U.S. Pat. No. 6,830,324 discloses an air-permeable film provided at one end of an air path to allow air to pass through, but not ink. The air path has an air introduction port in the ink container sized to form an ink meniscus that breaks to allow air into the reservoir when the pressure in the reservoir becomes sufficiently negative. However, it has been found that such an air introduction port would need to have an opening diameter of approximately 50 microns in order to operate satisfactorily for allowing air into the reservoir as described. A difficulty in providing such a small hole in an ink container is that ink containers are typically injection molded, and a 50 micron molding pin to provide such a hole in the container would not be sufficiently robust for injection molding large numbers of ink containers.
What is needed is an ink tank having a spring-biased flexible wall in which air can enter the ink reservoir at a more gradual rate to keep the regulated pressure more uniform. In addition, a method is needed for making such an ink tank in a low cost way that is compatible with high-volume manufacturing methods.